b 1. Field of the Invention
The present invention relates in general to boiling water reactors and more specifically to an apparatus and method of construction for fuel bundles of boiling water reactors.
2. Related Art
Fuel bundles for boiling water reactors typically each provide a plurality of vertically stacked fuel rods. Common fuel bundles or fuel assemblies provide a square or rectangular shaped perimeter wall called a channel within which the fuel rods are positioned. Reactor coolant flowing through the boiling water reactor enters the bottom of the channel and passes vertically upward and longitudinally over the fuel rods where it is heated to form steam. The steam discharges from upper openings in the fuel bundle. Boiling water reactors may contain several hundred fuel bundles. One or more water passages are also commonly provided within fuel bundle assembly to maintain a source of water to slow down a sufficient quantity of neutrons to initiate and maintain reactor criticality.
The highest potential operating energy capability for a boiling water reactor is obtained if all fuel rods are full-length fuel rods. The disadvantage of using 100% full-length fuel rods is that reactor shut-down margin is not optimized. Following a reactor shut-down, fission does not immediately stop. Neutrons continue to fission, and it is necessary to trap sufficient neutrons to prevent inadvertent reactor criticality. Shut-down margin is therefore a sufficient percentage of trapped neutrons compared to fissioned neutrons which prevents criticality. Shut-down margin is commonly enhanced by distributing a quantity of part-length fuel rods in each fuel bundle. A vacant volume above each part-length fuel rod provides an additional water volume when the reactor is shut down. These additional water volumes trap neutrons to provide increased shut-down margin for the reactor. Common boiling water reactor fuel bundles have part-length fuel rods evenly distributed about the fluid flow channels but within an outer ring of full-length fuel rods.
The known configurations of part-length fuel rods in a fuel bundle do not achieve optimum reactor critical power and/or shut-down margin.